According to several sources, there are 30 to 50 million people in the world who are lactose intolerant. In the 1960s and 1970s, it was reported that 70% of the adults in the world had lactose intolerance. In 1995, it was reported that 75% of the adults in the world and 25% of the adults in the U.S. were categorized as being lactose intolerant. In 1994, it was reported that 75% of African Americans and Native Americans and 90% of Asian Americans had lactose intolerance. It has also been reported that 30% of adults who are mostly North American descendants of Europeans have adapted to high lactase activity into adulthood. Research concludes that this adaptation is genetically controlled, permanent, and related to a long tradition of milk and milk product consumption in these regions of the world.
Lactose intolerance is the inability to digest significant amounts of lactose, a major natural sugar found in milk and milk products of all mammals. Lactose intolerance is caused by a shortage of the enzyme lactase, which is produced by the cells that line the small intestine and is essential to lactose digestion. Lactase breaks down lactose, a disaccharide, into two simpler forms of sugar called glucose and galactose, which are then transported across the cell membrane and absorbed into the bloodstream. If lactase is not present, or not present in sufficient levels, excess undigested lactose passes through the small intestines into the large intestine where it is fermented by bacteria in the colon (“colonic microbiota,” “gut microbiota,” “intestinal microbiota,” or “commensal gut microbiota”). The fermentation of lactose in the large intestine produces hydrogen and methane which can lead to bloating, gas, and diarrhea. These symptoms are caused by a very low activity of lactase in the intestines and are found in subjects who are lactose intolerant. Not all subjects deficient in lactase have the symptoms commonly associated with lactose intolerance, but those who do are said to have lactose intolerance.
If a subject suspects that he or she has lactose intolerance, it is potentially harmful for him or her to restrict his or her diet because restriction can result in a nutrition shortage or a failure to detect a more serious disease. Milk and other dairy products are major sources for nutrition in the basic American diet. The primary nutrients in milk are protein, calcium, riboflavin, vitamin A, and vitamin D. Calcium is an important part of the recommended daily allowance of vitamins and minerals and any deficiency therein can lead to health risks such as osteoporosis, hypertension, or weak bone density.
Young children who have lactose intolerance are very rare. The amount of the enzyme lactase a body produces generally reaches a maximum immediately after birth and then decreases in the majority of people during the ages of about 3-15.
Generally, humans develop lactose intolerance from a primary or secondary cause. The primary cause is an onset of loss of lactase that is believed to be a permanent condition. This onset can occur at a variable period after the weaning period. The primary cause is also genetically determined. The secondary cause is generally a temporary condition that occurs as a result of another disease or event that damages the lining of the small intestine where lactase is active. This temporary condition can be caused by acute diarrhea, disease, parasitic infection, Cohn's disease, celiac disease, gastrointestinal surgery, or the intake of certain medications.
In addition to the primary and secondary causes, certain human ethnic and racial populations have more of a predisposition for lactose intolerance. In these populations, social and cultural habits and attitudes influence lactose intolerance. Lactose activity can also decrease with age in certain ethnic and racial populations, including those populations which have origins in Europe, the African plains, and the Siberian Steppes. Humans who are most likely to have or develop lactose intolerance include those of Asian, Middle Eastern, North American, African, and Latin American decent.